Control system for DC motors

ABSTRACT

In a DC motor control system, switching element driving circuits are connected to respective base circuits of four switching elements arranged to form a bridge circuit which has a DC motor connected thereto as an electric load thereon, and an output signal interlocking circuit is provided in each paired driving circuits associated with the corresponding paired switching elements which are connected in series across a DC power source, respectively, thereby preventing simultaneous conduction of each paired switching elements. By virtue of the provision of the output signal interlocking circuits, it is possible to realize a PWM control system for DC motors with a simplified circuit construction and yet to thereby prevent power source short-circuiting trouble with elevated safety and reliability.

This invention relates to control systems for DC motors, and more particularly to a control system which controls a DC motor used in a robot, a machine tool or the like according to the technique of pulse width modulation (which will be abbreviated hereinafter as PWM).

In order to give clear understanding of the construction and operation of a conventional control system, an explanation will hereinafter be made of an exemplifying prior art control system by making reference to FIGS. 1 to 4.

A system having the structure shown in FIG. 1 is known as one of prior art control systems of this kind. FIG. 2 shows an exemplified practical construction of power transistor driving circuit 13 shown in FIG. 1. FIG. 3 shows modes of switching four power transistors Tr₁ to Tr₄ constituting a bridge circuit supplying a power supply voltage V_(D) to a DC motor 18 shown in FIG. 1, and FIG. 4 shows operating waveforms at various parts of the electric circuit shown in FIG. 1.

Referring to FIG. 1, a speed command signal a is externally applied to a subtractor 1 which subtracts a speed feedback signal b from the externally applied speed command signal a, and a speed amplifier 2 amplifies an output signal c from the subtractor 1 to convert it into a current command signal d. A second subtractor 3 subtracts a current feedback signal e from the current command signal d to generate an output signal f, and a current amplifier 4 amplifies the input signal f to generate a control signal g. A triangular waveform generating circuit 5 generates a triangular waveform signal h. A third subtractor 6 subtracts the triangular waveform signal h from the control signal g to generate an output signal n₁, and an adder 7 adds the triangular waveform signal h to the control signal g to generate an output signal n₂. A comparator 8 compares the input signal n₁ with a set voltage ΔV and generates an output pulse j₁ when the level of the input signal n₁ is higher than +ΔV, and a second comparator 9 compares the input signal n₁ with a set voltage -ΔV and generates an output pulse j₂ when the level of the input signal n₁ is lower than -ΔV. A third comparator 10 compares the input signal n₂ with the set voltage -ΔV and generates an output pulse j.sub. 3 when the level of the input signal n₂ is lower than -ΔV, and a fourth comparator 11 compares the input signal n₂ with the set voltage ΔV and generates an output pulse j₄ when the level of the input signal n₂ is higher than +ΔV. An interface circuit 12 converts the input signals j₁, j₂, j₃ and j₄ into output signals k₁, k₂, k₃ and k₄, respectively, and provides electrical isolation between the input signals and the output signals. Power transistor driving circuits 13, 14, 15 and 16 convert the input signals k₁, k₂, k₃ and k₄ into drive signals m₁, m₂, m₃ and m₄, respectively. Power transistors Tr₁, Tr₂, Tr₃ and Tr₄ constitute a bridge circuit for applying a power supply voltage V_(D) from a DC power source 17 to a DC motor 18, and flywheel diodes D₁, D₂, D₃ and D₄ are connected in parallel with the power transistors Tr₁, Tr₂, Tr₃ and Tr₄, respectively. A speed sensing unit such as a tachogenerator 19 is directly coupled to the DC motor 18 to generate the speed feedback signal b proportional to the rotation speed of the DC motor 18. A current sensing unit 20 generates the current feedback signal e proportional to an electric current i_(M) supplied to the DC motor 18.

Referring to FIG. 2, reference numeral 38b designates a phototransistor on the secondary side of a photocoupler, which is a part of the interface circuit 12 and converts the input signal j₁ to the interface circuit 12 into the signal k₁ ; 31 designates a DC power source of the driving circuit 13; and Tr₅, Tr₆ and Tr₇ designate transistors.

Power transistor switching modes in the prior art control system having such structure will be described with reference to FIG. 3. Although the flywheel diodes D₁ to D₄ are not shown in FIG. 3 for simplicity of illustration, it should be understood that such diodes are connected in parallel with the power transistors Tr₁ to Tr₄, respectively.

In a mode 1, the voltage applied across the DC motor 18 is null (zero), and due to a voltage induced by the rotating DC motor 18, an electric current i_(M) flows through the power transistor Tr₁ or Tr₃ and an associated flywheel diode. At (a) of the mode 1, the power transistor Tr₁ is turned on; at (b) of the mode 1, the power transistor Tr₃ is turned on; and at (c) of the mode 1, the power transistors Tr₁ and Tr₃ are turned on, but with all of the other power transistors remaining nonconductive.

In a mode 2, the voltage applied across the DC motor 18 is also null, and due to a voltage induced by the rotating DC motor 18, an electric current i_(M) flows through the power transistor Tr₄ or Tr₂ and an associated flywheel diode. At (a) of the mode 2, the power transistor Tr₄ is turned on; at (b) of the mode 2, the power transistor Tr₂ is turned on; and at (c) of the mode 2, the power transistors Tr₂ and Tr₄ are turned on, but with all of the other power transistors remaining nonconductive.

In a mode 3, the power transistors Tr₂ and Tr₃ are turned on, and the power supply voltage V_(D) is applied across the DC motor 18 to cause the current i_(M) to flow therethrough.

In a mode 4, the power transistors Tr₁ and Tr₄ are turned on, and the power supply voltage V_(D) is applied across the DC motor 18 to cause the current i_(M) to flow therethrough. In a mode 5, all of the power transistors Tr₁ to Tr₄ are turned off.

The operation of the prior art control system shown in FIG. 1 and that of the driving circuit 13 shown in FIG. 2 will be described with reference to FIG. 4 showing operation waveforms at various parts.

The output signal c of the subtractor 1 indicative of the result of the subtraction of the speed feedback signal b from the externally applied speed command signal a is amplified by the speed amplifier 2 which generates the current command signal d. Then, the output signal f of the subtractor 3 indicative of the result of the subtraction of the current feedback signal e from the current command signal d is amplified by the current amplifier 4 which generates the control signal g. The triangular waveform signal h generated by the triangular waveform generating circuit 5 has a constant amplitude of ±Vh and a constant period T₀. The triangular waveform signal h is subtracted from the control signal g in the subtractor 6 which generates the output signal n₁, and the signal h is added to the control signal g in the adder 7 which generates the output signal n₂.

The signal n₁ is applied to the comparators 8 and 9. When the relation n₁ >+ΔV holds between the input signal n₁ and the set voltage ΔV, the output pulse j₁ of a high level is generated from the comparator 8, while, when the relation n₁ <-ΔV holds, the output pulse j₂ of a high level is generated from the comparator 9. On the other hand, when the relation -ΔV≦n₁ ≦+ΔV holds between n₁ and ΔV, both of the output pulses j₁ and j₂ have a low level. The time interval between the pulses j₁ and j₂ is ΔT. The signal n₂ is applied to the comparators 10 and 11. Similarly, when the relation n₂ >+ΔV holds between the input signal n₂ and the set voltage ΔV, the output pulse j₄ of a high level is generated from the comparator 11, while, when the relation n₂ <-ΔV holds, the output pulse j₃ of a high level is generated from the comparator 10. On the other hand, when the relation -ΔV≦n₂ ≦+ΔV holds between n₂ and ΔV, both of the output pulses j₃ and j₄ have a low level, and the time interval therebetween is also ΔT.

The interface circuit 12, which inputs the signals j₁ to j₄, acts to provide electrical isolation between its inputs and outputs so that a switching noise caused by the power transistors Tr₁ to Tr₄ may not adversely affect the operation of the electric circuits in the preceding stages. Upon receipt of the input signals j₁ to j₄, the interface circuit 12 generates the output signals k₁ to k₄, respectively.

Referring to FIG. 2 showing an exemplified practical construction of the driving circuit 13, the output signal k₁ from the phototransistor 38b on the secondary side of the photocoupler forming a part of the interface circuit 12 is applied to the driving circuit 13, and it is amplified by the transistors Tr₅ and Tr₆, and the driving circuit 13 generates the output pulse signal m₁ for driving the power transistor Tr₁. In FIG. 2, the transistor Tr₇ is turned on in response to the turning-off of the transistor Tr₅, and, when the transistor Tr₇ is turned on, it applies a reverse voltage across the base-emitter junction of the power transistor Tr₁, thereby shortening the turn-off recovery time of the power transistor Tr₁. The remaining driving circuits 14 to 16 operate similarly. The drive signals m₁ to m₄ drive the power transistors Tr₁ to Tr₄, respectively. Here, the operation mode differs depending on the level of the control signal g. FIGS. 4 (a), 4 (b) and 4 (c) show the operation waveforms at various parts of the electric circuit shown in FIG. 1 when the magnitude of the control signal g varies to become g=0, g=ΔV and g=V₁ (V₁ >ΔV), respectively. It will be seen from FIG. 4 (c) that, when g>ΔV holds, the operation mode becomes the mode 3 in which the power transistors Tr₂ and Tr₃ are simultaneously turned on and apply across the DC motor 18 a voltage having a period T₀ /2, an amplitude -V_(D) and a pulse width T_(a). Then, when the control signal g, which is expressed by g=V₁, is applied, the mean voltage V_(M) applied across the DC motor 18 is represented by the following equation: ##EQU1##

It can be seen from the above equation that the mean voltage V_(M) applied across the DC motor 18 can be linearly controlled from zero to -V_(D) while the magnitude of V₁ changes from ΔV to (Vh+ΔV).

The operation mode is constituted by the repetition of an operation cycle which comprises the transition in the order of the mode 1→mode 3→mode 2→mode 3, and there flows through the DC motor 18 the current i_(M) having a repetition frequency which is two times that of the triangular waveform signal h. Though not shown, when the control signal g is of the magnitude represented by g<-ΔV, the operation mode is the mode 4 in which the power transistors Tr₁ and Tr₄ are simultaneously turned on to apply the powcr supply voltage +V_(D) across the DC motor 18. Then, when the control signal g, which is expressed by g=V, is applied, the mean voltage V_(M) applied across the DC motor 18 is represented by the following equation: ##EQU2## It can be seen that the mean voltage V_(M) applied across the DC motor 18 can be linearly controlled from zero to +V_(D) while the magnitude of V₁ changes from -ΔV to (-V_(h) -ΔV). The operation mode is constituted by the repetition of an operation cycle which comprises the transition in the order of the mode 1→mode 4→mode 2 →mode 4, and there flows through the DC motor 18 the current i_(M) having a repetition frequency which is two times that of the triangular waveform signal h. On the other hand, when the control signal g is of the magnitude represented by 0≦g≦ΔV, there is no mode in which both of the drive signals m₂ and m₃ for driving the respective power transistors Tr₂ and Tr₃ have a high level, and hence no voltage is applied across the DC motor 18. Similarly, when the control signal g is of the magnitude represented by -ΔV≦g≦0, there is no mode in which both of the drive signals m₁ and m₄ for driving the respective power transistors Tr₁ and Tr₄ have a high level, and hence no voltage is applied across the DC motor 18. Thus, when the magnitude of the control signal g is in the range of -ΔV≦g≦ΔV, no voltage is applied across the DC motor 18, and such a range forms an uncontrollable region which is called a dead zone. It is therefore necessary to decrease the detection level ΔV of the comparators as far as possible to the extent allowable for the circuit operation.

In the drive signals m₁, m₂ or m₃, m₄ for driving the power transistors Tr₁, Tr₂ or Tr₃, Tr₄ connected in series across the DC power source 17, respectively, it is necessary to provide the time period ΔT during which both of such signals m₁, m₂ or m₃, m₄, respectively, have a low level. This is because, even after the drive signal m₁ has become a low level, the power transistor Tr₁ may be turned on if a forward voltage is applied thereacross within the turn-off recovery time t_(stg) of the power transistor Tr₁. By selecting the length of ΔT to be larger than the maximum length of t_(stg), it is possible to have the power transistor T_(r1) turned off completely before the drive signal m₂ having a high level is applied to the power transistor T_(r2), so that there is no mode in which both of the power transistors Tr₁ and Tr₂ are in the conductive state. As seen from FIG. 4, the time period ΔT in which both of the drive signals m₁ and m₂ have a low level is provided by the values of the set voltages +ΔV and -ΔV of the comparators 8 and 9, respectively, and, in this time period ΔT where the input signal n₁ is -ΔV≦n₁ ≦ΔV, both of the pulses j₁ and j₂ are at a low level. The signals j₁ and j₂ are converted into the signals k₁ and k₂ through the interface circuit 12, respectively, and the signals k₁ and k₂ are then converted into the drive signals m₁ and m₂ through the driving circuits 13 and 14, respectively. However, the value of ΔT may vary, if there is involved any time delay in the conversion processes. As described above, it is necessary to shorten the length of ΔT, on one hand, in order to decrease the dead zone, and to lengthen it, on the other hand, in order to prevent a short-circuit of the power source due to simultaneous conduction of the power transistors. Therefore, it has been necessary in the prior art to select ΔV and ΔT of suitable minimum values so that the delay time involved in the processes for converting the signals j₁ to j₄ into the signals m₁ to m₄, respectively, may be minimized. Thus, the prior art control system is disadvantageous in that expensive high quality photocouplers having a quick response have to be used in the interface circuit 12, and yet, it is impossible to prevent a breakdown of the power transistors from being caused by the simultaneous conduction of the power transistors and the resultant short-circuiting of the power source which are due to possible inclusion of any faulty component in the driving circuits.

With a view to removing such prior art drawbacks as mentioned above, it is a primary object of the present invention to provide an improved DC motor control system having a simplified circuit construction in which inexpensive elements are used to constitute the interface circuit and a breakdown of the power transistors caused by any faulty component involved in the circuit can be prevented.

In order to attain the above object, the DC motor control system according to the present invention comprises a first comparator and a second comparator for generating an output pulse signal when the results of the subtraction and addition of a triangular waveform signal h from and to a control signal g are higher and lower than zero volt, respectively, a first, seconds, third and fourth driving circuits to which an output of the first comparator, its inversion output, an output of the second comparator and its inversion output are applied respectively though an interface circuit, and output interlock circuits provided in the first and second driving circuits and in the third and fourth driving circuits, respectively, whereby the output signals of the respective driving circuits are used to drive power transistors forming a bridge circuit, thereby controlling a DC motor.

The present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing the circuit construction of a prior art DC motor control system;

FIG. 2 is a circuit diagram showing the practical structure of one of the driving circuits in the prior art control system;

FIG. 3 shows switching modes of the bridge circuit constituted by the four power transistors in the prior art control system;

FIG. 4 shows operation waveforms at various parts of the electric circuit shown in FIG. 1;

FIG. 5 is a block diagram showing the circuit construction of the DC motor control system of a preferred embodiment of the present invention;

FIG. 6 is a circuit diagram showing the practical structure of two of the driving circuits in the DC motor control system of the present invention; and

FIG. 7 shows operation waveforms at various parts of the electric circuits shown in FIGS. 5 and 6.

The DC motor control system of a preferred embodiment of the present invention will now be described with reference to FIGS. 5 to 7. FIG. 5 is a block diagram showing the circuit construction of the DC motor control system embodying the present invention, FIG. 6 is a circuit diagram showing the practical structure of power transistor driving circuits 25 and 26, and FIG. 7 shows operation waveforms at various parts of the electric circuits shown in FIGS. 5 and 6. In FIGS. 5, 6 and 7, same reference numerals are used to designate the same or equivalent parts appearing in FIGS. 1, 2, 3 and 4.

Referring to FIG. 5, a speed feedback signal b is subtracted from an externally applied speed command signal a in a subtractor 1, and an output signal c of the subtractor 1 is amplified by a speed amplifier 2 which generates a current command signal d. A current feedback signal e is subtracted from the current command signal d in a subtractor 3, and an output signal f of the subtractor 3 is amplified by a current amplifier 4 which generates a control signal g. Then, a triangular waveform signal h generated by a triangular waveform generating circuit 5 is subtracted from the control signal g in a subtractor 6 which outputs a signal n₁. At the same time, the signals h and g are added in an adder 7 which outputs a signal n₂. The signals n₁ and n₂ are applied to comparators 29 and 30, respectively. The comparator 29 generates an output pulse signal p₁ of a high level when the level of the input signal n₁ is higher than zero volt. The output pulse signal p₁ is applied to an interface circuit 24, and at the same time it is applied to an inverter 22 which generates an inverted output signal p₂ which is applied to the interface circuit 24. The comparator 30 generates an output pulse signal p₃ of a high level when the level of the input signal n₂ is lower than zero volt. The output pulse signal p₃ is applied to the interface circuit 24, and at the same time it is applied to an inverter 23 which generates an inverted output signal p₄ which is applied to the interface circuit 24.

Upon receipt of the input signals p₁, p₂, p₃ and p₄, the interface circuit 24 generates output signals q₁, q₂, q₃ and q₄ which, in turn, are converted into power transistor drive signals r₁, r₂, r₃ and r₄ through driving circuits 25, 26, 27 and 28, respectively. The drive signals r₁, r₂, r₃ and r₄ respectively drive power transistors Tr₁, Tr₂, Tr₃ and Tr₄, which are connected to form a bridge circuit. Further, as shown in FIG. 6, the output signal q₁ from a secondary phototransistor 36b of a photocoupler forming a part of the interface circuit 24 is applied to the driving circuit 25 to turn on transistors Tr₈ and Tr₉ when the signal q₁ is at a high level, thereby supplying a current to a primary photodiode 34a of a photocoupler 34 in the driving circuit 25. At this time, a transistor Tr₁₀ is in the conductive state, and the transistor Tr₁₀ is turned off upon turning-off of a secondary phototransistor 35b of a photocoupler 35 in the driving circuit 26, whereby the drive signal r₁ of a high level is generated from the driving circuit 25 to turn on the power transistor Tr₁. When the primary photodiode 34a of the photocoupler 34 is turned on, the secondary phototransistor 34b of the photocoupler 34 is turned on, which causes a transistor Tr₁₃ in the driving circuit 26 to conduct and hence the drive signal r₂ to disappear. Then, when the output signal q₂ from a secondary phototransistor 37b of a photocoupler forming a part of the interface circuit 24 turns to a high level, transistors Tr₁₁ and Tr₁₂ are turned on to supply a current to a primary photodiode 35a of the photocoupler 35 in the driving circuit 26. As a result, the secondary phototransistor 35b of the photocoupler 35 is turned on, which causes the transistor Tr₁₀ to conduct and hence the drive signal r₁ to disappear.

Then, when the signal q₁ ceases to be applied to the driving circuit 25, the transistors Tr₈ and Tr₉ are turned off and the current supplied to the primary photodiode 34a of the photocoupler 34 stops flowing, thereby causing the secondary phototransistor 34b of the photocoupler 34 to be turned off and hence a transistor Tr₁₃ to become nonconductive. As a result, the drive signal r₂ of a high level is applied to the base of the power transistor Tr₂ to turn the same conductive. Thus, the photocouplers 34 and 35 form an interlock circuit interlocking the drive signals r₁ and r₂. Reference numerals 32 and 33 designate DC power sources for the driving circuits 25 and 26, respectively.

The photocouplers used to constitute the interface circuit and interlock circuit are selected from among those whose output has a short rise time of the order of 1 to 2 μsec and a long fall time of the order of about 10 to 20 μsec, only the fall time being selected to be substantially the same among the photocouplers used. Such photocouplers are easily available and inexpensive.

Similarly to the prior art control system, in the DC motor control system of the present invention having the above-described construction, the repetition frequency of the current i_(M) flowing through the DC motor 18 is twice that of the triangular waveform signal h. The voltage induced in the DC motor 18 is not fed back to a DC power source 17, as shown by the power transistor switching modes 1 and 2, but causes a current to flow through a closed circuit loop formed by the power transistors, flywheel diodes and DC motor. The power transistor switching modes of the control system of the present invention are the same as those of the prior art control system so as to minimize current ripples and ensure an operation with high efficiency and a low noise.

An explanation will be made hereunder with reference to FIG. 7, which shows operation waveforms at various parts of the electric circuits shown in FIGS. 5 and 6, as to assured provision of the time period ΔT during which both of the power transistor drive signals r₁, r₂ and both of the power transistor drive signals r₃, r₄ are not generated.

FIG. 7 shows a case where the control signal g is zero volt. Since the value of ΔT is not affected by the value of g as shown in FIG. 4 illustrating the operation of the prior art control system, the following description will be made with respect to the case of g=0 by way of example.

The output signal p₁ of the comparator 29 and the inverted signal p₂ thereof do not simultaneously have a low level. The output signals q₁ and q₂ from the photocouplers, which respectively form parts of the interface circuit 24 and receive the signals p₁ and p₂ as input signals thereto, have a negligibly short rise time. However, the signals q₁ and q₂ fall being delayed by a time period ΔT₁ as compared with the fall of the signals p₁ and p₂, respectively. Therefore, during the time period ΔT₁, both of the signals q₁ and q₂ are at a high level. Since the delay time of the transistors Tr₈ and Tr₉ in the driving circuit 25 and that of the transistors Tr₁₁ and Tr₁₂ in the driving circuit 26 are negligible, the conducting durations of the transistors Tr₈ and Tr₉ are equal to the high level time period of the signal q₁, and the conducting durations of the transistors Tr₁₁ and Tr₁₂ are equal to the high level time period of the signal q₂. The secondary phototransistors 34b and 35b of the photocouplers 34 and 35, respectively, are turned on simultaneously with the turning-on of the transistors Tr₉ and Tr₁₂, respectively, but the former are turned off with a delay time ΔT₂ from the turning-off of the latter, respectively. The conducting durations of the transistors Tr₁₀ and Tr₁₃ are respectively equal to those of the secondary phototransistors 35b and 34b of the photocouplers 35 and 34, respectively. The drive signal r₁ is generated when the transistors Tr₉ and Tr₁₀ are turned on and off, respectively, and the drive signal r₂ is generated when the transistors Tr₁₂ and Tr₁₃ are turned on and off, respectively. Therefore, the time period ΔT, during which both of the drive signals r₁ and r₂ are at a low level, is given by (ΔT₁ +ΔT₂), and there is no mode in which both of the power transistors Tr₁ and Tr₂ become conductive at the same time.

Similarly, the photocouplers in the driving circuits 27 and 28 form an interlock circuit, so that the time period ΔT, during which both of the drive signals r₃ and r₄ are at a low level, is given also by (ΔT₁ +ΔT₂), and there is no mode in which both of the power transistors Tr₃ and Tr₄ become conductive at the same time.

Further, when the control signal g is zero, there is no mode in which both of the power transistors Tr₁ and Tr₄ or both of the power transistors Tr₂ and Tr₃ respectively become conductive at the same time, and the commonly nonconducting time period of the two power transistors, respectively, is similarly ΔT. Therefore, the range of the value of g represented by -ΔV₁ ≦g≦ΔV₁ provides the dead zone.

As will be understood from the foregoing descriptions, the present invention can realize a PWM control system for DC motors having a simplified circuit construction in which the dead zone in the PWM shaping circuit is eliminated to simplify its circuit construction, the cost of the interface circuit employing photocouplers or the like is reduced by the use of electric signals having no dead zone, and the output signal interlocking circuits, which are constituted by photocouplers and inherently provided with a dead zone, are disposed in the final switching element driving circuits thereby to provide complete nonconduction time period for paired switching elements connected in series across a DC power source, whereby power source short-circuiting trouble caused by simultaneous conduction of such paired switching elements can be prevented, even if a failure should occur in any circuit component and the like.

In the foregoing description of the embodiment of the present invention, the power transistors Tr₁ to Tr₄ have been referred to as a typical example of switching elements. However, it is apparent that the power transistors may be replaced by MOS FET's or the like. 

We claim:
 1. A DC motor control system comprising:a DC motor; a bridge circuit which is constituted by first, second, third and fourth switching elements and which is fed by a DC power source and has said DC motor connected thereto as an electric load thereon; a speed sensor for sensing a rotational speed of said DC motor; a current sensor for sensing an electric current flowing through said DC motor; a triangular waveform generator for generating a triangular waveform signal; an adder for adding the triangular waveform signal to a control signal obtained on the basis of an externally applied speed command signal and negative feedback signals supplied from said speed sensor and said current sensor; a subtractor for subtracting the triangular waveform signal from the control signal; a first comparator for generating an output signal when an output signal of said adder, which is applied to said first comparator, is at a level lower than zero volt; a second comparator for generating an output signal when an output signal of said subtractor, which is applied to said second comparator, is at a level higher than zero volt; first and second driving circuits to which the output signal of said first comparator and an inverted signal thereof are applied respectively through an interface circuit; and third and fourth driving circuits to which the output signal of said second comparator and an inverted signal thereof are applied respectively through said interface circuit, said first and second driving circuits and said third and fourth driving circuits being provided with respective output signal interlocking circuits, whereby said third and fourth switching elements connected in series across said DC power source are driven by said first and second driving circuits, respectively, and said first and second switching elements connected in series across said DC power source are driven by said third and fourth driving circuits, respectively.
 2. A DC motor control system as claimed in claim 1, wherein said switching elements are power transistors.
 3. A DC motor control system as claimed in claim 1, wherein said interlocking circuits are constituted by photocouplers.
 4. A DC motor control system as claimed in claim 1, wherein said switching elements are power transistors, and said interlocking circuits are constituted by photocouplers.
 5. A DC motor control system comprising:a speed sensor for sensing a rotational speed of a DC motor; a current sensor for sensing an electric current flowing through said DC motor; a triangular waveform generator for generating a triangular waveform signal; an adder for adding the triangular waveform signal to a control signal obtained on the basis of an externally applied speed command signal and negative feedback signals supplied from said speed sensor and said current sensor; a subtractor for subtracting the triangular waveform signal from the control signal; a first comparator for generating an output signal when an output signal of said adder, which is applied to said first comparator, is at a level lower than zero volt; a second comparator for generating an output signal when an output signal of said subtractor, which is applied to said second comparator, is at a level higher than zero volt; first and second driving circuits to which the output signal of said first comparator and an inverted signal thereof are applied respectively through an interface circuit; third and fourth driving circuits to which the output signal of said second comparator and an inverted signal thereof are applied respectively through said interface circuit; third and fourth switching elements connected in series across a DC power source which are driven by said first and second driving circuits, respectively, and first and second switching elements connected in series across said DC power source which are driven by said third and fourth driving circuits, respectively; and a DC motor connected across output ends of a bridge circuit constituted by said first, second, third and fourth switching elements, said first and second driving circuits and said third and fourth driving circuits being provided with respective output signal interlocking circuits.
 6. A DC motor control system comprising:a DC motor; a bridge circuit which is constituted by first, second, third and fourth power transistors and which is fed by a DC power source and has said DC motor connected thereto as an electric load thereon; a speed sensor for sensing a rotational speed of said DC motor; a current sensor for sensing an electric current flowing through said DC motor; a triangular waveform generator for generating a triangular waveform signal; an adder for adding the triangular waveform signal to a control signal obtained on the basis of an externally applied speed command signal and negative feedback signals supplied from said speed sensor and said current sensor; a subtractor for subtracting the triangular waveform signal from the control signal; a first comparator for generating an output signal when an output signal of said adder, which is applied to said first comparator, is at a level lower than zero volt; a second comparator for generating an output signal when an output signal of said subtractor, which is applied to said second comparator, is at a level higher than zero volt; first and second inverters for receiving the output signals generated by said first and second comparators and producing inverted output signals, respectively; first and second driving circuits to which the output signal of said first comparator and the output signal of said first inverter are applied as input signals thereto, respectively, through an interface circuit comprising photocouplers therein; and third and fourth driving circuits to which the output signal of said second comparator and the output signal of said second inverter are applied as input signals thereto, respectively, through said interface circuit, said first and second driving circuits and said third and fourth driving circuits being provided with respective output signal interlocking circuits, whereby said third and fourth power transistors connected in series across said DC power source are driven by said first and second driving circuits, respectively, and said first and second power transistors connected in series across said DC power source are driven by said third and fourth driving circuits, respectively. 